Golf club having a hollow pressurized metal head

ABSTRACT

A golf club having a hollow golf club head which is filled with a gas under pressure. The interior surface of the golf club head is coated with a solidified layer of plastic material. The pressurized gas permits the use of thinner face plates by compensating for forces generated when the face plate strikes a golf ball. The plastic layer is preferably applied through the process of rotational molding using a thermoplastic material.

STATEMENT OF RELATED APPLICATION

This application claims priority to provisional U.S. Patent ApplicationNo. 60/899,951, filed Feb. 7, 2007, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to golf equipment and more particularlyto a golf club having a hollow club head filled with fluid underpressure so that the club head may have a face plate which willwithstand deflection during impact with the golf ball.

2. Background of the Art

It is well known that all golfers struggle to improve their scoringduring a game of golf. As part of this, changes in golf clubmanufacturing have improved the technology to allow golfers to utilizedrivers and fairway metal woods to gain a longer distance off the tee ordown the fairway. As examples of the improved technology, such golfclubs are available at the present time which are lighter in weight, areimpact resistance, are manufactured from titanium or stainless steel,have shafts which are tailored to various swing speeds, have increasedhead size and the like.

It is also recognized among golfers that with a given club the golferhaving a faster swing speed will generate more distance than a golferhaving a slower swing speed. As a result, even though there has beenvast improvements in golf club head technology to assist golfers noattempt has been made (other than shaft design) to allow a golfer with alower swing speed to achieve the benefits of the improved golf club headtechnology in a manner which has occurred with the golfers having muchhigher swing speeds. At the same time if golf club head technology canallow golfers with lower swing speeds to achieve longer distance with aclub, such technology also may be utilized to allow the golfers withhigher swing speeds to achieve an even greater distance than isavailable with technology at the present time.

Therefore, it would be desirable to provide a golf club with a golf clubhead manufactured using technology that would allow greater distance andaccuracy while remaining within the design criteria limits establishedby the United States Golf Association (USGA). Such golf clubs would bedrivers, fairway clubs, hybrid clubs and irons.

SUMMARY OF THE INVENTION

A hollow metal golf club head having the interior thereof filled with agas under pressure and having the interior surface thereof coated with athermoplastic material acting as a sealant adapted to prevent thepressurized gas from passing through pores formed in the golf club head.The golf club head includes a valve which is disposed within a cavityformed at the time of manufacture of the club head preferably in thelower rear portion of the sole of the club head and which includes aspring-loaded member which is sealed against a surface within the cavityand which member may be moved away from its sealed position to allow gasunder pressure to enter the hollow interior of the club and thenrestored to the sealed position to retain the gas under pressure withinthe hollow interior of the club.

A method of manufacturing a golf club head including providing a hollowgolf club head having an interior surface, inserting particles of athermoplastic material into the hollow interior, heating the club headto a temperature equal to at least the melting point of thethermoplastic material, rotating the head, while maintaining thetemperature thereof, simultaneously through a vertical and a horizontalaxis to coat the interior surface thereof with the melted thermoplasticmaterial, and cooling the club head to solidify the thermoplasticmaterial.

In accordance with a further aspect of the present invention theinterior surface of the hollow club head is covered by a plasticmaterial which is coated upon the interior surface of the club head byrotational molding to thus apply a continuous plastic surface that willseal the interior surface of the club head to prevent the escape of thegas under pressure contained therein.

In accordance with yet a further aspect of the present invention hollowmetal golf club heads including metal woods and irons having face plateson the order of 1.0 millimeters to 7.0 millimeters containing acompressed gas therein having a pressure on the order of 20 to 300pounds per square inch are provided and tailored for swing speedsbetween 40 mph and 160 mph.

In accordance with yet another aspect of the present invention the faceplate of a hollow pressurized golf club head may have regions thereofdisplaced from the central striking zone of the face plate reduced inthickness to provide a greater “sweet spot” or several “sweet spots” onthe club face, each being supported by the pressurized gas.

In accordance with an additional aspect of the present invention, thereis provided a gas charging system which is connected between a source ofgas under pressure and a valve contained within a hollow metal golf clubhead which includes an actuator for moving the valve from a closed to anopen position to permit gas under pressure to fill the hollow interiorof the golf club and to allow the valve to move from an open to a closedposition to retain the gas under pressure within the hollow golf clubhead.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a golf club head constructed inaccordance with the principles of the present invention;

FIG. 2 is a cross-sectional view of the golf club head of FIG. 1 takenabout the lines 2-2 of FIG. 1.

FIG. 3 is an exploded view showing the structural components of thevalve incorporated into the head as shown in FIG. 1;

FIG. 4 is a cross-sectional view of a valve incorporated into a golfclub head in accordance with the principles of the present invention;

FIG. 4A is a partial cross-sectional view of an alternative embodimentof the valve;

FIG. 5 is a perspective view of a retainer used in such valve;

FIG. 6 is a cross-sectional view of the retainer shown in FIG. 5 takenabout the lines 6-6 thereof;

FIG. 7 is a perspective view of a valve stem used in the valve of thepresent invention;

FIG. 8 is a plan view of the valve stem as shown in FIG. 7;

FIG. 9 is a block diagram illustrating a system for pressurizing thegolf club head of the present invention;

FIG. 10 is a perspective view of a charging system connected to the golfclub head;

FIG. 11 is a plan view of the charging system as shown in FIG. 10;

FIG. 12 is a cross-sectional view of the charging system shown in FIG.11 taken about the lines 12-12 thereof; and

FIG. 13 is an exploded view showing the various components of thecharging system used in accordance with the present invention;

FIG. 14 is a bottom view of a driver showing the valve and a protectivecover.

DETAILED DESCRIPTION

The present invention is an improvement over the golf club having ahollow air filled head as disclosed and claimed in U.S. Pat. No.6,019,687 issued Feb. 1, 2000 to Alden J. Blowers, one of theco-inventors named herein, which by this reference is incorporatedherein insofar as the same is not contrary to the disclosure and claimsrelating to the present invention.

To accommodate golfers desiring to hit the golf ball a further distancewith drivers and fairway woods, the golf manufacturing technology hasprovided metal golf club heads commonly referred to as metal woods. Thetechnology has progressed through the utilization of forged metal faceplates which are fitted to molded metal bodies. Such bodies may beformed from titanium or steel and the face plates may likewise be formedfrom titanium or steel depending upon the particular application.Typically, a titanium molded body must be utilized to receive a titaniumforged face plate. The USGA has imposed specific limitations as to thesize of the head, the coefficient of restitution (COR) of the faceplate, the moment of inertia and the like. Typically, the coefficient ofrestitution for a club face on a driver cannot exceed 0.830 and thevolume of the driver cannot exceed 460 cubic centimeters with atolerance of plus 10 cc. It has been found when the club face is reducedto a thickness below approximately 2.8 millimeters that a golfer havinga high swing speed (for example, in excess of 95 to 100 mph) can damagethe club face by bending it or in some instances if the club head speedis fast enough actually fracturing the club face upon impact of the ballon the club face. On the other hand, it has also been determined that asthe club face gets thinner, the golfer is able to generate more velocityof the golf ball leaving the club face at the same club head speedthereby causing the ball to travel farther. It is for this reason thatthe COR maximum was instituted by the USGA.

At the present time, little or no attention has been paid to the averagegolfer who has a swing speed substantially less than the 90 mph which isusually the lower limit of the better golfers. For example, the typicalwoman golfer will have a club head speed on the order of 55 to 60 mphand junior golfers may have a swing speed starting at 40 mph. With sucha club head speed utilizing the 2.8 millimeter thickness of the typicalclub face will not produce the desired results which are generallysought after and achieved only by the golfers having the high club headswing speeds. There is therefore needed a system whereby a thinner clubhead face, on the order of 1.0 to 2.2 millimeters, may be used whichwill allow the golfer having a low club head speed to generate greaterdistance and to achieve the benefits of the present technology. However,such a thin club face even at lower club head speeds can still generateproblems with the COR and potential damage to the club face.

By utilizing the principles of the present invention and pressurizingthe internal volume of the club head by utilization of a compressed gas,the forces generated on the club face by the club head striking the golfball even with a very thin club face can be compensated for, thusproviding the ability for the golfer to utilize the thinner club face,thus generating more ball velocity off the club face but at the sametime not damaging the club face or violating the COR restrictions.

Referring now to the drawings and more particularly to FIGS. 1 and 2,there is illustrated a golf club head made in accordance with theprinciples of the present invention. Illustrated in FIG. 1 is a metalwood such as a driver which is typically used by a golfer to drive agolf ball a long distance off the tee. The metal wood golf club 10includes a head 12 having a hosel 14 and a club face 16. A shaft 18 isconnected to the hosel 14 and is gripped by the golfer to manipulate theclub 10 to cause the club face 16 to strike a golf ball and propel itdown the fairway. When a golf club such as that shown in FIG. 1 is usedto strike a golf ball, the golf ball stays in contact with the club face16 approximately 450 micro-seconds and upon impact exerts an averageforce of approximately 2000 lbs. on the golf ball. Typically a golf clubhead of the type shown in FIG. 1 is manufactured from titanium orstainless steel utilizing a metallic casting. A club face 16 isgenerally formed separately from the remainder of the club head and isthen welded in place on the club head 12. The club face may be forged,although such is not required. Typically, if the club face is of forgedtitanium, the club head 12 will be formed from cast titanium so that themetals are compatible for welding. Similarly, if the forged club face 16is made of stainless steel, the club head 12 will also be made of caststainless steel. It should be recognized that the entire club head maybe formed by casting without departing from the scope of the presentinvention.

As above indicated, the technology of the invention herein disclosed hasresulted in the ability to make the club face 16 thinner and to allowgreater force to be applied to the golf ball causing it to travelfurther when it is impacted by the club head. However, as the club face16 gets thinner, the large amount of force exerted upon impact with thegolf ball can destroy the club face 16. To preclude this occurring, theclub head 12 is hollow as illustrated in FIG. 2 and in accordance withthe principles of the present invention, the hollow interior 20 isfilled with compressed gas having sufficient pressure to support thethin club face. Various compressed gases including air may be utilized,however, in accordance with a preferred embodiment of the presentinvention nitrogen is utilized. Nitrogen is preferred because themolecules of nitrogen are larger in size than many other gases and thuswill not as easily migrate through the pores in the cast club head aswould gases having smaller molecules.

In accordance with the principles of the present invention the interiorsurface 22 of the hollow club head 12 is covered with a sealant 24 tofurther preclude the compressed gas from escaping through the pores inthe cast material. As is also illustrated in FIG. 2, the opening fromthe hollow interior of the club head into the hosel 14 is plugged at thetime of formation by a plug 26 which preferably is formed as a integralpart of the casting of the club head. The coating 24 on the interiorsurface used to seal the club head 12 may be formed by rotationalmolding using a thermoplastic resin. Any thermoplastic resin which willadhere to the interior surface of the hollow club head and which willexpand and contract with movement of the club head as a result oftemperature changes may be utilized. For example, cross linked or highdensity polyethylene may be used and in accordance with a preferredembodiment of the present invention, a polyurethane resin functionsadequately. Rotational molding takes place by melting a thermoplasticresin in powder or pellet form in a bi-axially rotating heated mold. Inaccordance with the present invention the hollow club head could be themold. Alternatively, a plurality of hollow club heads, each containingthe granular thermoplastic resin, may be supported internally of achamber on a frame which is bi-axially rotated or the entire heatedchamber may be rotated with the heads mounted thereon. The particles ofthermoplastic resin melt and puddle in the bottom of the hollow clubhead. As the club head is rotated simultaneously through a vertical anda horizontal axis, the interior surface of the club head passes throughthe puddle of thermoplastic material causing a thin layer of thematerial to coat the inner surface of the hollow club head and fusethereto in layers. This process continues with the fused layer becomingprogressively thicker until the desired wall thickness of the coating onthe interior surface of the hollow club head is achieved. Under somecircumstances the interior surface of the hollow club head may need tobe cleaned, such as degreasing, prior to the rotational molding. Suchcleansing assures adherence of the plastic coating to the club head. Byproviding such a coating on the interior surface 22 of the hollow clubhead 12 all of the pores which may exist in the molded club head 12 areclosed or covered thereby retaining the pressurized gas internally ofthe club head without substantial leakage.

By referring now to FIGS. 3 and 4 there is illustrated a valve and themanner in which it is retained within the club head 12. The valve isutilized to pressurize the hollow interior 20 of the club head 12. FIG.3 illustrates an exploded view of the components of the valve. The valveincludes a spring 30, a valve core or plunger 32, an o-ring 34, and aretainer or body 36. The spring 30 engages a surface 38 on the stem 32while the o-ring 34 is situated within the groove 40 of the stem 32. Theo-ring engages a surface 42 formed on a bore through the retainer 36 andeffectuates a seal to retain the pressurized gas within the hollowinterior 20 of the club head 12. The manner in which the componentsshown in FIG. 3 are retained within the club head is illustrated in FIG.4. The structure as shown in FIG. 4 may be formed at any positiondesired within the club head but for a driver it is preferred to be atthe center rear thereof adjacent to or on the sole to assist inperformance of the club as to launch angle and center of gravity. Theclub head 12 at the time of molding is formed with a cavity 44 whichincludes an opening 46 through which compressed gas passes. The interiorsurface 48 of the initial portion of the opening 44 is threaded andreceives the external threads 50 formed on the retainer 36. The threadsmay be tapered or straight. The threads 50 mate with the threads on thesurface 48 in such a manner than an airtight seal is formed. The spring30 is seated against the bottom portion 52 of the cavity 44. The stem 32is deposited on the end 54 of the spring 30. When the stem 32 ispositioned on the end 54 of the spring 30, it is pushed downwardly byinserting the retainer 36 into the cavity 44 and engaging the threads 50with the threads on the interior surface 48 of the cavity. The retaineris then threaded until it is seated in position as shown in FIG. 4. Whensuch is done, the o-ring 34 will form the seal between the stem 32 andthe surface 42 of the retainer 36. The bore 37 in the retainer 36 isthreaded as shown at 53 to receive a tool (not shown) to assist inthreading the retainer 36 into the cavity.

FIG. 4A illustrates a preferred alternative embodiment of the valve asinstalled in the golf club head. The structure is substantially the sameas shown in FIG. 4 and above described except for the retainer 36. Asillustrated in FIG. 4A, the retainer 39 includes an unthreaded extension41 which defines a groove 43 therein. Seated within the groove 43 is anadditional “O” ring 45 which seats against the wall 47 of the cavity 49.The additional “O” ring 45 is an added safety feature to prevent leakageof the pressurized

To insert gas under pressure into the hollow interior 20 of the clubhead 12 the valve stem is moved from the position shown in FIG. 4 (theclosed position) against the force of the spring 30 to allow gas to passby the stem 32 and through the opening 46 into the hollow interior 20 ofthe club head 12. When the gas has reached the desired pressure, thestem is allowed to return to the position shown in FIG. 3 and once againto seal the cavity 44 to preclude the pressurized gas from exiting thehollow interior 20 of the club head 12.

Referring now more particularly to FIGS. 5 and 6, the retainer 36 isshown in greater detail. As is therein illustrated, the retainer 36having the threads 50 formed on the external surface thereof defines abore 80 therethrough. As is shown in FIG. 6 the initial portion of thebore 80 has the threads 53 formed therein. The lower portion of the bore80 has a larger diameter than does the initial portion and receives thehead 82 of the stem 32 as is illustrated in FIG. 4.

In FIGS. 7 and 8 the stem is shown in greater detail. As is thereinshown, the stem 32 having the groove 40 formed therein includes the head82 and a flange 84. The surface 86 of the flange abuts the lower surface88 of the retainer 36 and is retained in contact therewith by the forceof the spring 30. The groove 40 is formed between the head 82 and theflange 84 of the stem 32 and as above described receives the o-ring 34for sealing against the surface 42 of the lower portion of the bore 80of the retainer 36.

By referring now more particularly to FIG. 9 there is shownschematically a system for pressurizing the hollow interior 20 of theclub head 12. As is therein illustrated, there is provided an actuator60 which includes means 62 for attaching the actuator to the valve 64which in turn is inserted into the cavity 44 of the hollow club head 66as shown in FIG. 4. As is illustrated in FIG. 5, a gas source 68 isaffixed to the actuator 60 as illustrated at 70. An appropriate gauge 72is also attached to the actuator 60 to monitor the pressure which isbuilt up inside the hollow club head 66. To pressurize the hollow clubhead 66 the actuator is attached to the internal threads 53 on theretainer 36 and the gas source 68 is then attached to the actuator 60.Thereafter, the actuator 60 is manipulated in such a manner that aplunger enters the retainer 36 and engages the surface 78 of the stem 32to move it from its closed position as illustrated in FIG. 4 to an openposition thus allowing the gas from the source 68 to pass through thevalve 64 into the interior of the hollow club head 12. When the gauge 72indicates that the correct amount of pressure has been generated toproperly pressurize the hollow interior 20 of the club head 12, theactuator is deactivated to allow the valve to return to its closedposition as illustrated in FIG. 4. The hollow club head is then removedfrom the actuator and the pressurized golf club is ready forutilization.

Although any apparatus desired by one skilled in the art which willfunction in accordance with the flow diagram shown in FIG. 9 and theabove description may be utilized, one form of such a fixture isillustrated in FIGS. 10 and 13 to which reference is hereby made. As istherein shown, the fixture includes a body 90 having a cap 92 whichreceives an actuator rod 94. An o-ring 96 is utilized to provide a sealbetween the cap 92 and the body 90 when the cap is secured thereto. Astop 98 cooperates with the actuator 94 and the body 92 as will bedescribed more fully below. A recess or flat 100 is provided in the body90 and an orifice 102 is defined therein. An additional o-ring 104 isutilized to seal the body 90 to the valve as is illustrated more fullyin FIG. 12.

FIG. 10 illustrates the structure as shown in FIG. 13 assembled andattached to the valve which is disposed within the recess 44 formed inthe club head 12 which is schematically represented in FIG. 10. Themechanism described above is illustrated further in FIG. 11 in a planview thereof. It is also illustrated in further detail in FIG. 12 whichis a cross-sectional view taken about the lines 12-12 of FIG. 11 andillustrates in greater detail the manner in which the apparatus orfixture is attached to the valve 64. As is therein shown, the body 90defines a bore 106 therethrough within which the actuator rod 94 isdisposed. The cap 92 is threadably received within the body 90 and it issealed therein by the o-ring 96. The stop 98 extends through the rod 94and is disposed to reciprocate within a slot 108 formed within the cap92. As is illustrated, the body 90 is affixed to the retainer by way ofthe threads 110 formed thereon which engage the threads 53 formed in theupper portion of the retainer 36. The source of gas 68 may be attachedto the opening 112 while a gauge may be attached to the opening 114 inthe body 90.

In operation the source of gas under pressure attached to the body 90would be open to permit gas to enter the bore 106 within the body 90.The actuator rod would be manipulated toward the left as shown in FIG.12 by applying a force F to the end 116 thereof. The rod would then movewithin the slot 108 toward the left until it bottomed out at the endthereof which would cause the valve stem 32 to move toward the left asshown in FIG. 12 thereby disengaging the o-ring from the internalsurface of the retainer 36. When such is done, gas would flow from thesource thereof through the bore 106 and into the hollow interior of thegolf club. When the desired pressure as indicated by the gauge isreached, then the force F would be removed and the spring 30 wouldreturn the stem 32 to the position shown in FIG. 12 at which point thegas source would be removed and the fixture as shown in FIG. 13threadably removed from the club head. At this time the internal hollowvolume of the club head is filled with gas at the desired pressure. Ifdesired, a protective cap (not shown) may be secured in place over thevalve entrance to prevent tampering or other manipulation of the valvedisposed in the sole plate of the club head.

FIG. 14 illustrates a driver club head which has the valve asabove-described located at the rear thereof adjacent the sole plate. Thehead 130 has a face 132 and a rear 134 with a soleplate 136. Extendingfrom the head is the hosel 138. A valve 140 is affixed to the lower rearportion of the head adjacent the rear of the sole plate 136. Aprotective member such as a cover 142 or cap is affixed to the soleplate and surrounds the valve. The protective member prevents the userfrom tampering with the valve. Although the protective cover 142 isshown as a cylinder, it should be understood that it can take anygeometric form desired.

By utilization of the pressurization system above described hollow golfclub heads having face plates of relatively minimum thickness on theorder of 1.0 to 2.8 millimeters may be pressurized at various pressuresto counteract the forces generated by the face plate contacting a golfball at various club head speeds. It will be understood by those skilledin the art that these thinner face plates are supported by thecompressed gas housed in the hollow club head to provide maximumperformance for the golfer while still remaining within the limitationsset forth by the regulations of the USGA. As an example, if a golfer'smeasured swing speed is 60 to 65 mph, then utilizing a 2.2 millimeterclub face thickness, the pressurized gas internally of the hollow clubhead would be less than 150 lbs. per square inch. On the other hand, asthe club head speed generated by the golfer increases, the amount ofpressure internally of the hollow club head would increase to supportthe thinner face while permitting maximum performance of the club faceto obtain the benefits of the present technology. Such a process wouldcontinue until a golfer having a club head speed exceeding 90 mphnecessitates the pressure internally of the hollow club head to begreater than 150 psi and preferably would be between 150 and 300 psi tosupport the thinner club face and thus preclude damage to it even thougha greater amount of force is generated upon impact of the club face withthe golf ball. It will be recognized by those skilled in the art thatthrough utilization of a system where club head speed is correlated toclub face thickness and internal pressurization of the hollow club head,golfers may be fitted with the proper club to provide the greatestperformance for each golfer irrespective of club head speed.

Utilizing the principles of the present invention a hollow club headhaving a face plate that will compensate for off center strikes may beaccomplished. Areas of reduced thickness of the face plate may be formeddisplaced from the center of the face plate toward the heel, toe, top orbottom thereof. These reduced thickness areas allow the off centerstrike to still be in a so-called “sweet spot” of the club face therebyproviding better performance of the golf club even with off-centerstrikes. The reduced thickness areas are supported by the compressed gashoused internally of the hollow club head.

Since the advent of metal hollow clubs, particularly drivers, it hasbeen recognized by the golfers that a rather loud metallic sound occurswhen the club head strikes the ball. As club heads have gotten largerand larger generating a greater hollow interior space, this sound hasincreased dramatically. With the advent of the new square shaped clubheads the sound created by the impact of the club head against the ballis even further enhanced. It has been determined that this increasedsound is somewhat disconcerting to the golfers and efforts are beingmade to mitigate that sound. It has been discovered that throughutilization of the principles of the present invention and bypressurizing the interior hollow cavity of the club head with acompressed gas, particularly the driver, that the sound generated uponimpact of the club head with the ball is substantially mitigated. Whenthe thermoplastic coating is applied to the interior surface of thehollow club head, the sound generated is even further mitigated.

Although the foregoing description has been made with emphasis ondrivers and fairway woods, it is to be understood that the principles ofthe present invention are equally applicable to the hybrid clubs andalso to irons which may be manufactured with a cavity between the clubface and the rear of the club. By utilization of the principles of thepresent invention a thinner club face may be utilized on the irons thusproviding an enhanced performance of the irons.

There has thus been disclosed a hollow metal golf club head having theinterior thereof filled with a gas under pressure which includes a valvedisposed within a cavity formed at the time of the manufacture of theclub head and which may be reciprocated between a sealed and openedposition to allow gas under pressure to enter the hollow interior of theclub. The interior surface of the club is covered with a solid plasticmaterial to seal pores which may occur within the material from whichthe hollow club head is formed. Also disclosed is a system for chargingthe hollow interior of the club head with pressurized gas to therebyallow the fitting of golf clubs having face plates on the order of 1.1millimeter to 2.8 millimeters in thickness with compressed gas havingpressure on the order of 20 to 300 lbs. per square inch to tailor thegolf club to swing speeds between 40 mph and 160 mph.

What is claimed is:
 1. A golf club comprising: a hollow club head havingan interior surface; a shaft affixed to said club head; a gas underpressure filling said hollow club head; and a layer of solidifiedplastic material coating said interior surface of said hollow club head.2. A golf club head as defined in claim 1 wherein said plastic materialis coated by rotational molding.
 3. A golf club head as defined in claim2 wherein said plastic material is polyurethane.
 4. A golf club head asdefined in claim 2 wherein said plastic material is polyethylene.
 5. Agolf club head as defined in claim 2 wherein said gas is nitrogen.
 6. Agolf club head as defined in claim 1 which further includes a meansaffixed to said club head for injecting said gas under pressure intosaid hollow club head.
 7. A golf club head as defined in claim 5 whereinsaid means for injecting said gas under pressure is a valve affixed tothe rear of said club head.
 8. A golf club head as defined in claim 2wherein the pressure of said gas is 20 to 300 pounds per square inch. 9.A golf club head as defined in claim 7 which further includes aprotective member at said valve to prevent tampering with said valve.10. A method of manufacturing a golf club head comprising: providing ahollow golf club head having an interior surface; injecting a granularthermoplastic material into said hollow golf club head; heating saidgolf club head to a temperature at least equal to the melting point ofsaid thermoplastic material; rotating said golf club head simultaneouslythrough a vertical and horizontal axis while maintaining saidtemperature to thereby coat said interior surface with saidthermoplastic material; and cooling said golf club head to solidify saidthermoplastic material.
 11. The method as defined in claim 10 whichfurther includes filling said hollow golf club head with a gas underpressure.
 12. The method as defined in claim 11 wherein said gas isnitrogen.
 13. The method as defined in claim 11 wherein the pressure ofsaid gas is 20 to 300 pounds per square inch.
 14. The method as definedin claim 10 wherein said thermoplastic material is polyurethane.
 15. Themethod as defined in claim 10 wherein said thermoplastic material iscross-linked polyethylene.
 16. The method as defined in claim 10 whereinsaid thermoplastic material is high density polyethylene.
 17. The methodas defined in claim 11 which further includes providing a valve having aplunger movable between open and closed positions carried by said hollowgolf club head, moving said plunger to its open position to fill saidhollow head with gas and moving said plunger to its closed position. 18.The method as defined in claim 17 which further includes measuring thepressure of said gas during filling of said hollow head.